The present invention relates to a spring-loaded driving member for a timepiece movement comprising two coaxial barrels coupled in series, each barrel having a rotatable cylindrical housing provided with an outer tooth arrangement and a base, and a spring rolled up inside the housing and having an outer end coupled to the housing and an inner end coupled to a core, wherein the respective cores of the barrels are interdependently rotatable and the respective bases of the barrels are located axially opposite one another. In other words, the two barrels are superposed and face one another, an arrangement that is represented in particular in patent applications EP 1 115 040 (with two groups of two barrels) and EP 1 657 604.
In the field of mechanical watch movements it is well known to replace the usual driving member comprising a single barrel fitted with a spring by a group of two barrels coupled in series in order to store a sufficiently large amount of potential energy to assure a power reserve greater than the usual 40 hours or so without having an effect on the chronometric performances of the watch or the efficiency of the mechanism. A detailed explanation of the functional characteristics of such a driving member may be found in the Swiss patent CH 610 465 which provides a superposed arrangement and a side-by-side arrangement of the barrels as examples. In the present case, it is the superposed arrangement that is selected because the moment can be transmitted from one barrel to the other directly via a common shaft, which prevents losses of space and efficiency as a result of the transmission gearing that is necessary in the side-by-side arrangement.
However, the coaxial and superposed arrangement of the usual barrels, as may be seen in the abovementioned patents, results in the driving arrangement having a relatively large space requirement in axial direction, i.e. in the direction perpendicular to the main bottom plate of the timepiece movement. In fact, the total height of the device is not only made up of twice the respective heights of the base, spring, cover and internal clearance of each barrel, but additionally a minimum gap between the two covers that face one another and must not touch one another, since they rotate at different speeds. Consequently, the total height of an arrangement with superposed barrels always gives rise to a relatively large thickness of the watch movement and this constitutes a disadvantage in most cases. Since the number of expansion turns of a barrel spring cannot be increased without reducing its efficiency, if the manufacturer wishes to reduce the height of the barrels and therefore of the springs, he automatically reduces the energy that can be stored and thus also the power reserve of the watch.